When drilling into hard earth, such as rock, it is necessary to produce an impact force at the penetrating end of the drilling tool in order to crush the rock for increasing the boring rate. Typically, a pressurized fluid, such as compressed air, is used as the energy source in creating the impact force. As will be appreciated, the greater the fluid pressure, the greater the rate of boring. However, with an increase in fluid pressure also comes an increase in the cost of pressurizing the fluid, which can be substantial for large drilling operations.
The prior art percussion drilling tools can be categorized in two general classes. The first class is a high-efficiency type which is characterized by efficient consumption of the pressurized fluid supplied to produce the percussive effects. The high-efficiency type drillling tools, however, operate only with relatively low fluid pressure input due to the inherent structurally weak piston designs associated therewith. The pistons of the high-efficiency type drilling tools are structurally weak because they necessarily include a network of internal flow passages for conducting the pressurized fluid between both ends of the piston depending on the position of the piston in the cylinder. That is, the pistons themselves act as a control valve alternately conducting the pressurized fluid between opposite ends of the piston depending on the position of the piston relative to the cylinder, thereby forcing the piston to move in either direction. Therefore, the high-efficiency type perrcussion drilling tools are ineffective for high-rate drilling, i.e., high-pressure fluid supply, due to their inherently weak piston structures.
The second class of percussion drilling tools is the high-operating pressure type. The high-operating pressure type characteristically provide high-rate drilling due to their structurally sound piston designs, and thus their capability to utilize high pressure fluid. That is, the high-operating pressure type pistons do not include a network of internal flow passages extending therethrough as do the high-efficiency type pistons. However, the high-operating pressure type percussion drilling tools are inherently inefficient in their consumption of pressured fluid. Because the pistons of this class, like those of the high-efficiency class, act as their own control valve by controlling the fluid flow with the changing position of the piston within the cylinder, there necessarily results pressure loss during the impact stroke when the pressurized fluid is suddenly conducted to the opposite end of the piston. This pressure loss results in the drilling tools of the high-operating pressure type being deficient for economical high-rate drilling.
The U.S. Pat. No. 3,332,504 to Lowery issued Jul. 25, 1967, discloses an impact tool including a spool-type control valve for alternately conducting a flow of pressurized fluid between opposite ends of a piston. The control valve is reciprocated between two fluid-conducting positions by the same fluid pressure that urges the piston to move. The Lowery impact tool is deficient in that movement of the control valve between its two fluid-conducting positions is dependent upon the position of the piston. That is, while the control valve is in a position for conducting fluid to the upper end of the piston to force the piston toward an impact position, the control valve remains stationary. When the piston is prevented from moving further, i.e., when the piston is in its impact position, the fluid pressure in the cylinder acts upwardly through a small vertical passage to move the control valve upwardly to a position for conducting fluid to the lower end of the piston. This results in a hesitation, or dwell, in the impact cycle as the piston stops, then the control valve moves, then the fluid is redirected to the opposite end of the piston. Therefore, the Lowery impact tool provides a relatively slow impact cycle as the movement, i.e., operation, of the control valve is dependent upon the position of the piston relative to the cylinder.
The U.S. Pat. No. 4,172,411 to Matsuda et al, issued Oct. 30, 1979, discloses a hydraulic hammer including a spool valve for directing hydraulic fluid pressure to either end of a double-acting cylinder depending upon the position of a piston within the cylinder. The Matsuda patent, like the Lowery patent, is deficient in that the piston impact cycle is relatively slow due to the dependency of the spool valve movement upon the piston position.
The U.S. Pat. No. 4,179,983 to Wallace, issued Dec. 25, 1979, discloses a hydraulic hammer including a spool valve for directly hydraulic fluid pressure to a piston. The spool valve moves between two fluid directing positions in response to the position of the piston within its cylinder. The Wallace patent, like the Lowery and Matsuda patents, is deficient in that the piston impact cycle is relatively slow due to the dependency of the spool valve movement upon the piston position.
It will be appreciated from the foregoing description of the prior art that there exists a need in the percussion drilling tool art for a drilling tool which efficiently uses pressurized fluid, is structurally capable of high-rate drilling supplied by high-pressure fluid, and is capable of relatively rapid impact cycles.